The biochemical pathways associated with 5-hydroxytryptamine (5-HT, serotonin) metabolism in invertebrate (Aplysia) and mammalian (rat) nervous systems are being characterized. Stable isotopically labeled substrates are being utilized to act as tracers of those pathways, and in developing an assay for the activity of the enzyme, tryptophan-5-hydroxylase, involved in 5-HT metabolism. Both labeled and unlabeled chemical compounds are being analyzed by gas chromatography-mass spectrometry-selected ion monitoring and by scanning gas chromatography-mass spectrometry. Volatile derivatives of the compounds involved in 5-HT metabolism are utilized in the GC-MS analyses. These studies are being carried out on individual, identified serotonergic neurons in the Aplysia, and in small regions of the rat brain. Characterization of the possible neurotransmitter roles of other putative neurotransmitters, especially amino acids, in identified Aplysia neurons is also being pursued. The relationships between tryptophan levels and the rates of 5-HT biosynthesis are being studied, and the effects of perturbing catecholamine levels on 5-HT concentrations are being examined. BIBLIOGRAPHIC REFERENCES: T.M. Iliffe, D.J. McAdoo, C.B. Beyer and B. Haber, Amino Acid Concentrations in the Aplysia Nervous System: Neurons with High Glycine Concentrations, J. Neurochem., May, 1977, in press. D.J. McAdoo, The Retzius Cell of the Leech Hirudo Medicinalis, in "Biochemistry of Characterized Neurons", Pergammon Press, 1977, in press.